1. Field of the Invention
This invention relates to an electric probing-test machine for inspecting device patterns formed on semiconductor wafers in a process of manufacturing semiconductor devices such as LSIs, or to a probing-test machine for inspecting Liquid Crystal Devices (LCD) and electronic circuits such as printed circuit boards. More specifically, the invention concerns an electric probing-test machine, which has a cooling system which can avoid formation of frost on the object while testing the object under a low temperature condition.
2. Description of the Related Art
Semiconductor devices which are operated at low temperatures, e.g., HEMTs and Josephson elements, are being developed prosperously for the purpose of computer operation speed increase. A characteristics test on each device pattern under a low temperature condition is conducted in the last stage before individual semiconductor chips are diced from the wafer because the cost of test becomes enormous if the test is conducted after the dicing process. This test is called a wafer test, which is an electric test conducted for each device pattern between a wafer formation process and an assembly process, i.e., after pre-process such as pattern etching or coating with a protective film. The wafer test is done for the purposes of rejecting defective devices in the stage before the individual semiconductor chips are cut from the wafer and also improving the yield and reliability by feeding back the test results to the preceding process.
A wafer test system employed for the test basically consists of two units, i.e., an electric probing-test machine (also called wafer prober) and a tester. The two units are connected together via a measuring line. In the system, probes are contacted to respective bonding pads of a device pattern, and test complete signals and test fall signals are exchanged between the wafer prober and tester with test start command from a test control line.
Wafer probers are disclosed in many publications such as Japanese Patent Publication Sho 59-50942, Japanese Utility Model Laid-Open Sho 61-97839 and Japanese Utility Model Laid-Open Sho 61-88240. These wafer-probing machines, however, are used under room temperature conditions and are not suited for wafer tests under low temperature conditions.
The specifications of Japanese Utility Model Laid-Open 59-35876, Japanese Patents Laid-Open 55-44931, 58-220438, 59-19343, 59-41846 and 59-57444, and Japanese Patent Application 62-60581 disclose wafer probers, in which a main chuck for holding a wafer can be cooled at the time of the test.
However, since these wafer-probing machines have openings at various parts thereof. e.g., the opening for entry of a probe card, humid air outside the machine enters the test section. Therefore, when the main chuck is cooled, the humid air present near the main chuck is cooled quickly by radiation cooling, and moisture in the air condenses and dew adheres to and is frozen on the chuck top.
When such frost is formed on the chuck top, the wafer can no longer be supported at a predetermined height level, thus making it difficult to have a large number of probes be in correct contact with also a large number of bonding pads. In addition, the wafer is frozen to the chuck top, making it difficult to remove the wafer from the chuck top. Further, if the frozen wafer is forcibly removed from the chuck top, damage to the wafer is prone.
The conventional chuck top cooling system serves to circulate a coolant such as ethylene glycol liquid, which has been cooled by the refrigerator, from the heat exchanging tank to the heat exchanging jacket by the feed pump.
When the chuck top reaches a predetermined temperature, the refrigerator is stopped by the control means (CPU). The coolant is thus warmed up by sensible heat of surrounded air in a minute. When the chuck top is also warmed up over the predetermined temperature by the warmed-up coolant circulated, the refrigerator is made operative in response to information applied from the temperature sensor, thereby keeping the chuck top at the predetermined temperature.
In the case of the conventional chuck top, however, ethylene glycol liquid circulated is cooled by the refrigerator and it is difficult to continual start up and shut down of the refrigerator. Even if it is asked that the refrigerator is started up and shut down at a certain interval, the compressor will prevent this. In short, it is difficult to frequently start and stop the compressor.
The motor for driving the piston of the compressor has a small initial starting force. In addition, the supply of current to the motor is automatically shut down when the piston is brought under high pressure but it is again started when high pressure is not applied to the piston. It is therefore difficult to frequently start and stop the compressor.
Because of this response delay of the compressor, temperature fluctuation (or temperature rippling phenomenon) is caused on the surface of the chuck top. This temperature fluctuation on the surface of the chuck top is usually in a range of .+-.2.degree..about.3.degree. C. and the semiconductor wafer mounted on this chuck top cannot be therefore stably cooled.
Further, the chuck top is made of insulating material such as ceramics and it is therefore low in heat conductivity. This makes temperature in the central portion of the chuck top different from that at the peripheral portion thereof. It is therefore quite difficult to uniformly cool the whole of the chuck top.
In the case of the conventional cooling system, the pump is made so high in pressure as to supply a large amount of coolant to the heat exchange jacket. When the discharge pressure of pump is made high in this manner, the pressure in the pipe through which the coolant is circulated becomes higher than atmospheric pressure, thereby causing the piping to be broken or the coolant to be leaked through couplings and the like. Therefore, a large amount of coolant cannot be supplied to the heat exchange jacket. This makes it difficult to fully and accurately cool the chuck top.
In order to fully cool the chuck top, it is needed that the coolant (or non-freezing liquid) is fully circulated through the heat exchange jacket. When heat exchange is carried in the tank, the coolant is needed to circulate in the tank in such a manner that it flows adjacent to the cooling coil (or evaporator) connected to the refrigerator.
In the case of the conventional system, however, liquid resistance in the piping between the tank and the chuck top is so large that the amount of coolant circulated cannot be increased over a certain value. Therefore, a part of coolant freezes adjacent to the cooling coil and this causes that area of the coil which is contacted with the coolant circulated to be made insufficient, thereby lowering the heat exchanging rate in the tank.
In order to increase the amount of coolant circulated in the heat exchange tank, it is imagined that an agitator is added to the tank, but this will make the cooling system large in size and complicated in construction with the result of its costing high. In addition, the coolant will be exposed to heat created by a motor for the agitator and temperature control for the coolant will become complicated accordingly.